Main links for useful things relating to MCNP:

https://mcnp.lanl.gov/

User manuals:

https://laws.lanl.gov/vhosts/mcnp.lanl.gov/mcnp_manual.shtml

Here's a full text:

https://laws.lanl.gov/vhosts/mcnp.lanl.gov/pdf_files/la-ur-17-29981.pdf

Launch with 

>./mcnp6 N=SAMPLE

Or 

>./mcnp6 IP N=SAMPLE

for plot window